Nutrient and arsenic biogeochemistry of Sargassum in the western Atlantic

Pelagic Sargassum spp. forms important floating habitats for hundreds of associated organisms and supports fish nurseries, seabirds, and sea turtles. Since 2011, massive accumulations in the Great Atlantic Sargassum Belt (GASB) have inundated Caribbean and Florida coastlines, degrading nearshore ecosystems, harming wildlife, impacting tourism and public health, and complicating potential biomass utilization due to arsenic content. Several interconnected hypotheses have been proposed to explain the GASB, including altered wind-driven transport from the Sargasso Sea into the eastern North Atlantic and enhanced nutrient supplies from upwelling, vertical mixing, large river discharges (e.g., Amazon, Congo), and atmospheric deposition. However, the causes and controls of the seasonal to interannual variability of the GASB remain unresolved. A key underlying question is whether the exceptional Sargassum abundance in the GASB results from higher nutrient availability. The study aims to quantify the nutritional status of Sargassum across western Atlantic regions and link arsenic content to phosphorus limitation to inform understanding of nutrient sources and ecosystem impacts.

Prior work established nutrient limitation of oceanic Sargassum in its native Sargasso Sea habitat and proposed enhanced nutrient availability as a driver of recent blooms. Documented ecological and socioeconomic impacts of GASB events include seagrass and coral degradation, altered trophic structures, reduced turtle hatchling survival, and challenges to tourism and public health. Hypothesized nutrient sources span riverine inputs (notably Amazon influence), upwelling and mixing, and atmospheric deposition. Earlier studies have also characterized arsenic accumulation in Sargassum and its implications for biomass utilization, and satellite observations have traced Sargassum transport pathways and potential source regions. Collectively, the literature motivated testing whether nutrient enrichment characterizes GASB Sargassum and whether arsenic metrics can diagnose phosphorus stress.

Sampling occurred during R/V Thomas G. Thompson voyages TN389 (16 March–16 April 2021) and TN390 (20 April–16 May 2021), occupying GO-SHIP lines A20 and A22. Hydrography: Conductivity-Temperature-Depth (CTD) rosette profiles with Niskin bottle sampling were collected and nutrients analyzed at sea using a Seal Analytical AutoAnalyzer 3 following GO-SHIP protocols. Sargassum collection and identification: Surface Sargassum spp. were collected using a dip net and sorted by morphotype as S. natans I and S. fluitans III (with any morphologically similar S. natans VIII classified as S. fluitans III). Elemental and isotopic analyses: For each morphotype, up to three replicates (6–10 thalli/species) were briefly rinsed in deionized water, cleaned of macroscopic epibionts, dried at 65–70 °C for 48 h, powdered, and analyzed for carbon, nitrogen, phosphorus, arsenic content, and nitrogen stable isotopes (δ15N) using established analytical methods. Transport context: To assess recent origins, passive particles were initialized at each collection site and tracked backward 60 days in a numerical ocean model hindcast; ensemble trajectories and centroids provided likely source pathways. Environmental context: Hydrographic sections provided nitracline/phosphocline depths and near-surface nitrate and phosphate distributions to interpret Sargassum nutritional status across regions (Caribbean, western tropical Atlantic, Sargasso Sea, and northern Sargasso Sea).

- Sargassum elemental composition was similar between S. fluitans and S. natans across stations. Three regional regimes emerged: (1) Sargasso Sea with relatively high carbon and low nitrogen and phosphorus content; (2) GASB regions (Caribbean and western tropical Atlantic) with significantly higher nitrogen and phosphorus content than the Sargasso Sea; and (3) northern Sargasso Sea with the highest nitrogen and phosphorus content observed in the survey. - Elemental ratios reflected these trends: elevated C:N and C:P in the Sargasso Sea, with a zonal increase in C:N to the east (A20). N:P showed a pronounced maximum in the Sargasso Sea region of A22. - Hydrography showed deepest nitracline and phosphocline in the warm, salty Sargasso Sea and shallower nutriclines in the Caribbean and western tropical Atlantic; near-surface nitrate and phosphate were at or below detection in the Sargasso Sea but detectable in the Caribbean and western tropical Atlantic, consistent with enhanced Sargassum nutritional status in GASB regions. - δ15N patterns: Depleted δ15N in the Sargasso Sea, Caribbean, and western tropical Atlantic suggest inputs from nitrogen fixation and/or atmospheric deposition. Elevated δ15N (~+2‰) in the northern Sargasso Sea is consistent with influence from riverine sources (e.g., U.S. east coast, Mississippi via Gulf Stream) or from upwelled/vertically mixed nitrate with similar δ15N. Enriched δ15N was also observed in southern western tropical Atlantic samples under Amazon plume influence. - Arsenic biogeochemistry: Arsenic content was lowest in GASB regions and highest in the Sargasso Sea (especially on A20), mirroring low phosphorus content there. Consequently, Sargassum As:P ratios were uniquely high in the subtropical gyre where phosphorus limitation prevails. - Theory and observations: Based on Michaelis-Menten uptake kinetics and relatively uniform, uncorrelated surface arsenate versus phosphate, the As:P uptake ratio is predicted to vary hyperbolically with tissue phosphorus content. Observations from 2021 (N=200) together with historical datasets (1983–1987, N=20; 2015–2018, N=21) show As:P ∝ (%P)^{-1.3}, a supra-hyperbolic dependence statistically supported (p < 0.001), validating arsenic content as a diagnostic of phosphorus limitation in natural Sargassum populations. - Correlations: As content negatively correlates with P (and with N, via N–P covariation) and positively correlates with C; As:C does not show a hyperbolic relationship with carbon content.

The study demonstrates that GASB Sargassum is enriched in nitrogen and phosphorus relative to its Sargasso Sea habitat, indicating that nutrient supply is a primary driver of the widespread proliferation observed since 2011. δ15N patterns provide clues to nitrogen sources: depleted values are consistent with nitrogen fixation and/or atmospheric deposition, while enriched values in the northern Sargasso Sea and the southern western tropical Atlantic indicate episodic riverine inputs (e.g., Mississippi/Gulf of Mexico via Loop Current and Gulf Stream, Amazon plume) or upwelling/mixing in specific regions. The clear hyperbolic inverse relationship between arsenic-to-phosphorus ratio and phosphorus content, stronger than the simple theoretical inverse (exponent ≈ −1.3), firmly establishes arsenic content as an in situ indicator of phosphorus stress. This has practical significance: if phosphorus supply to the GASB diminishes relative to nitrogen in the future, arsenic levels in stranded biomass could rise toward Sargasso Sea levels, exacerbating risks for potential utilization and management. Distinguishing and quantifying the multiple nutrient sources that sustain the GASB will require spatially extensive, synoptic measurements of tissue nutrient contents and isotopes, integrated with transport modeling, to fingerprint sources and interpret variability on seasonal to interannual timescales.

This work provides basin-scale evidence that Sargassum within the GASB has enhanced nitrogen and phosphorus content compared with Sargasso Sea populations, implicating nutrient enrichment as a key factor behind the recent proliferation. It also establishes arsenic content—and specifically the hyperbolic As:P versus %P relationship—as a robust diagnostic of phosphorus limitation in natural Sargassum populations across decades of observations. These insights enable the use of tissue elemental and isotopic fingerprints to identify and distinguish nutrient sources (riverine, oceanic, atmospheric) that fuel the GASB. Future research should prioritize synoptic, basin-wide surveys of Sargassum elemental and isotopic composition, coupled with transport hindcasts/forecasts and biogeochemical modeling, to attribute sources, understand variability, and support improved seasonal to interannual predictions and management strategies.

- The specific sources and relative contributions of nutrients fueling the GASB remain unresolved; δ15N provides suggestive but not definitive attribution. - Strong seasonal to interannual variability of GASB extent and intensity complicates interpretation; synoptic sampling across the basin is challenging yet necessary for unambiguous source fingerprinting. - Particle backtracking indicates connectivity but is subject to model uncertainties and cannot uniquely determine source waters. - Some interspecies and station-level variability exists despite overall similar composition between S. fluitans and S. natans.